Project Summary There is a heart attack every 30 seconds in the United States. Despite receiving the best evidence-based care, many of these patients go on to develop heart failure, the most common reason for hospitalization. A fundamental problem is irreversible loss of cardiomyocytes that is replaced by cardiac fibrosis. Without a therapeutic option to solve a fundamental issue, the clinical prognosis of heart failure has to remain dismal. Among many other proposed strategies to regenerate heart muscle, the cardiomyocyte reprogramming approach is unique in that it uses resident heart cells to induce cardiomyocyte-like cells in the injured heart, obviating the need of transplanting cells into the heart. While the heart has very little regenerative capacity, it has a unique endogenous repair mechanism in response to injury. Loss of cardiomyocytes stimulates inflammatory responses to remove necrotic cardiomyocytes, and cardiac fibroblast activation leading to myofibroblast transdifferentiation. Myofibroblasts secrete excessive extracellular matrix proteins to form fibrotic scar which seals off the injured area where cardiomyocytes are lost. This fibrotic process initially protects the heart against infarct expansion and cardiac rupture, and maintains structural integrity. Thus, it remains unknown about how cardiac reprogramming, which has to coax some cells away from the endogenous infarct repair process, can enhance infarct repair without compromising the life-saving initial fibrotic process. Although fibrotic process is initially essential for survival after myocardial infarction, its persistence destroys myocardial architecture, thereby leading to cardiac dysfunction, adverse remodeling, and conduction abnormalities. Due to their detrimental effects in long-term and abundance in the injured area after the most vulnerable acute infarct period, myofibroblasts represent an attractive new target for cardiac reprogramming. We hypothesize that reprogramming scar-forming myofibroblasts after rupture-prone acute infarct period will enhance infarct repair by (a) redirecting cardiac fibrosis to new muscle formation, (b) inhibiting excessive or prolonged fibrogenic activation of myofibroblasts, and (c) minimizing the interference with the initial protective response. To test our hypothesis, we developed the new viral vector system that directs a transgene expression selectively in myofibroblasts to avoid targeting other cell types involved in infarct healing process. Using this novel tool set, our main objectives of this proposal are (a) to define how reprogramming the fate of scar-forming myofibroblasts enhances infarct repair and (b) to assess therapeutic safety and efficacy of this approach. This study may provide a unique platform to develop a new post-MI intervention targeting subacute or chronic phase of myocardial infarction for which no effective treatment is available.